The invention herein relates to fiber reinforced elastomeric materials. More particularly, it relates to rubber and rubber-like natural and synthetic elastomeric compositions reinforced with very small diameter glass fibers. The elastomeric materials reinforced in accordance with the present invention find use in various heated and moulded rubber products, such as gaskets, tire treads, sheet packings and the like.
It has heretofore been well known to use certain types of glass fibers as reinforcement for various plastic and rubber compounds. For instance, a number of patents which describe various rubber compounds to be used for sheet packing mention that the rubber can be reinforced by any of a variety of fibrous materials, such as asbestos and glass fiber. Also, various patents describe the use of strands or bundles of continuous glass fiber filaments for reinforcement of tires. In addition, the use of woven glass matts, fabrics and yarns for reinforcement of elastomers has been described. Heretofore, however, the short glass fibers of small diameter which comprise loose or "bulk" fiber or blown thermal insulating wool have not been considered suitable for reinforcement purposes for elastomers.
Similarly, there are few references to the use of fine diameter fibers as elastomer reinforcements. U.S. Pat. No. 3,556,844 shows continuous strand of glass fiber of 0.0001 to 0.0015 inch (2.5 to 38 microns) but does not deal with the use of short fibers.
While common reinforcing grades of short glass fibers have been successfully used to reinforce various types of thermoplastic and thermosetting resins, their use for elastomer reinforcement has been less widespread and considerably less successful. Such short "reinforcing" fibers are commonly considered to be the coarse D through U grades, which have fiber diameters of 0.0002 to 0.001 inch (5.1 to 25.4 microns). While the exact reasons for the relatively unsatisfactory performance of "reinforcing" glass fibers in elastomers are not precisely known, it has been observed that there is significant deterioration in properties with the use of ordinary glass fiber in the compounds. While certain properties, such as elongation, would be expected to be reduced because the inelastic glass fibers inhibit the stretching of the elastomer, it would normally be expected that such reduction and elongation would also be accompanied by a corresponding increase in tensile strength. Observations, however, have shown that such increase in tensile strength is not achieved in most cases, and where there is an increase it is not of a magnitude which would indicate that the short coarse glass fibers were in fact providing a satisfactory level of reinforcement. In short, therefore, the conventional short coarse "reinforcing" glass fibers have been found not to provide adequate reinforcement for elastomers as compared to materials such as asbestos fiber. Neither can they be considered to be functioning as fillers, for unlike conventional fillers such as carbon black the fibers cause serious deterioration in properties such as elongation without a concomitant increase in other properties.
In addition, there is believed to be a surface area factor involved in the adhesion of glass fiber to elastomers. For example, a unit weight of grade B glass fiber has approximately 80% more surface area than an equivalent unit weight of grade D fiber, and the disparity increases rapidly as larger fiber diameter grades are considered.
Various fiber lengths and diameters have been suggested for reinforcement of plastics. However, plastics and elastomers are of significantly different physical and chemical nature, and therefore, as noted above, teachings relating to plastics reinforcement have not normally been applicable to elastomer reinforcement. See, for instance, ASTM Special Technical Publication No. 184 (1956); Dannis, Rubber Age, pp 35-44 (July, 1975); and Rondeau, Machine Design, pp 154-163 (July 21, 1966).